Electronic pens are pen shaped devices that may serve as a data input apparatus for a computer or the like. Electronic pens operate as a standard pen permitting the user to write on paper while transmitting acoustic signals that are received by a receiver and used to determine the position of the pen. FIG. 1, by way of example, illustrates a basic electronic pen system 10 including a transmitter unit 12 that emits acoustic signals 14, and a receiver 20 that receives the acoustic signals 14. The transmitter unit 12 is in the form of a writing instrument, such as a pen, and includes a transducer 16 for generating acoustic signals 14, such as ultrasonic signals. The transmitter unit 12 may also include a transmitter 18, which transmits an infra-red (IR) or radio frequency (RF) signal 19 including telemetry data, such as switch status, pen ID, etc., and a time stamp. The receiver 20 includes two microphones 22a, 22bthat receive the acoustic signal 14, as well as an IR/RF receiver 24 that receives the IR/RF signal 19.
FIG. 2 illustrates the electronic pen system 10 attached to a computer 30. The receiver 20 is attached to a work area 40 and is also attached to the computer 30 via cable 32. The work area 40 is the area to which the writing instrument will physically write, which is illustrated in FIG. 2 as paper. The receiver 20 is attached to the work area 40 by a clip or other secure means so that the work area 40 and receiver 20 are not moved with respect to one another during use. The position of the work area 40 with respect to the receiver 20, as well as the work area 40 size, is defined through calibration. The position of the receiver 20 with respect to the work area 40, thus, does not change until the work area 40 is detached from the receiver 20. The receiver 20 receives the acoustic signals from the transmitter unit 12, from which the position of the transmitter unit 12 can be determined with respect to the receiver 20. The computer 30 maps the position of the transmitter unit 12 in a display 34.